The present invention pertains to improved electrostatic chucking devices used to secure a sample such as a semiconductor wafer in a microlithography apparatus. More specifically, the invention is directed to such devices that prevent decreases in microlithographic exposure accuracy and precision due to leakage of current from the chucking device to the wafer, thereby reducing wafer heating and consequent thermal distortion of the wafer.
A conventional electrostatic chucking device 51 is depicted schematically in FIG. 2. The device 51 is constructed of electrically insulative materials with interior electrically conductive electrodes 67, 69. The electrodes 67, 69 are electrically connected to a controlled power source 79 that supplies a voltage to each of the electrodes 67, 69. More specifically, the device 51 comprises a substrate 71 (normally made of a ceramic material) and a dielectric layer 61. The electrodes 67, 69 are situated between the substrate 71 and dielectric layer 61. The dielectric layer 61 includes a sample-contacting surface 60 parallel to the electrodes 67, 69.
The dielectric layer 61 typically has a thickness of a few hundred micrometers. Whenever a sample 3 (e.g., silicon wafer) is placed on the sample-contacting surface 60 and voltage is applied to the electrodes 67, 69, an electrostatic force, either a Coulomb force or a Johnsen-Rahbek force, is generated between the device 51 and the wafer 3. The force causes the wafer 3 to be electrostatically attracted to and thus held fast by the device 51.
Generally speaking, the Coulomb force dominates whenever the dielectric layer 61 has a large volume resistivity (e.g., greater than 1014 xcexa9-cm for Al2O3), and the Johnsen-Rahbek force dominates whenever the dielectric layer 61 has a small volume resistivity (e.g., within a range of 1010 to 1012 xcexa9-cm for Al2O3).
A Coulomb chucking force is a function of and substantially affected by the dielectric constant of the dielectric layer 61. Hence, to obtain a large Coulomb chucking force, the dielectric constant of the dielectric layer 61 should be correspondingly large. A Johnsen-Rahbek chucking force, in contrast, is a function of and substantially affected by the width of any gaps (microscopic or otherwise) between the sample-contacting surface 60 and the wafer 3. Hence, to obtain a large Johnsen-Rahbek chucking force, the sample-contacting surface 60 normally has a particular surface roughness.
Any of various problems can occur whenever a conventional electrostatic chucking device 51 is used to hold a sample (e.g., semiconductor wafer) in an electron-beam or other charged-particle-beam (CPB) microlithography (projection-exposure) apparatus. As noted above, if the volume resistivity of the dielectric layer 61 is relatively small, then the Johnsen-Rahbek force dominates and a strong chucking force can be obtained. However, whenever the volume resistivity of the dielectric layer 61 is relatively small, a voltage applied between the electrodes 67, 69 and the wafer 3 causes electrical current to flow to the wafer (e.g., 500 xcexcA current to 300-mm diameter wafer). The magnetic field generated from such a current can have a deleterious effect on a charged particle beam used for projection-exposure of a pattern to the wafer, which can result in a decreased accuracy of the exposed pattern.
On the other hand, as noted above, if the volume resistivity of the dielectric layer 61 is relatively high, then the Coulomb force dominates. Whenever the volume resistivity of the dielectric layer 61 is relatively high, very little current flows to the wafer 3. However, the ceramic material normally used to make the dielectric layer 61 generally has a relatively small dielectric constant, so a strong chucking force cannot be obtained. As a result, there is a substantial risk of the wafer 3 being displaced on or from the chucking device during microlithographic exposure. A principal cause of such detachment is thermal deformation of the wafer 3 caused by CPB-mediated heating of the wafer.
In view of the shortcomings of the prior art summarized above, an object of the invention is to provide electrostatic chucking devices, especially for use in microlithography, that exhibit improved resistance to decreases in exposure accuracy and precision due to current leakage to the sample (wafer) and/or displacement of the sample from thermal expansion.
To such end, and according to a first aspect of the invention, electrostatic chucking devices are provided to which a sample can be secured by an electrostatic chucking force. A representative embodiment comprises a chucking surface that comprises a dielectric layer. The dielectric layer is divided into at least a first region and a second region each formed of a respective dielectric material having a different respective dielectric property. The chucking device also comprises a respective electrode associated with each region. The first region desirably has a volume resistivity of 1014 xcexa9-cm or higher, and the second region desirably has a volume resistivity of 1013 xcexa9-cm or less. In addition, the first region desirably occupies at least 20% of the surface area of the chucking surface, and the second region desirably occupies no more than 30% of the surface area.
Alternatively, the first region can have a dielectric constant of 20 or higher, and the second region can have a dielectric constant of less than 20.
Each of the first and second regions of the dielectric layer can have multiple portions. The portions of both regions can be, for example, arranged concentrically with each other in ring-shaped configurations. Alternatively, for example, the second regions can be arranged as multiple point loci dispersed in the first region(s).
According to another embodiment, an electrostatic chucking device comprises an electrically insulative substrate, an electrostatic electrode situated on an upstream-facing surface of the insulative substrate, and a dielectric layer defining a sample-contacting surface. The dielectric layer is situated on the electrostatic electrode such that the electrostatic electrode is sandwiched between the insulative substrate and the dielectric layer. The dielectric layer comprises at least two coplanar portions at the chucking surface, wherein each planar portion is formed of a respective dielectric material having a different dielectric property than the other dielectric material.
In one configuration of this embodiment, the first region has a volume resistivity of 1014 xcexa9-cm or higher, and the second region has a volume resistivity of 1013 xcexa9-cm or less. In this configuration, the first region desirably occupies at least 20% of the surface area, and the second region desirably occupies no more than 30% of the surface area.
Alternatively, the first region can have a dielectric constant of 20 or higher, and the second region can have a dielectric constant of less than 20.
Further desirably, each electrode is configured so as to be energized with a voltage that can be independently controllable for each electrode. For example, the electrodes can be connected to a power source configured to energize the electrodes such that the sample held by the chucking device has a ground electrical potential.
The chucking surface can be roughed so as to have at least one indentation that is no more than 20 xcexcm deep relative to the chucking surface.
According to another aspect of the invention, methods are provided for holding a microlithographic sample. In a representative embodiment of such a method, an electrostatic chuck is provided that comprises a dielectric layer divided into at least a first and a second region each formed of a respective dielectric material having a different respective dielectric property. A respective electrode is provided for each region. The electrodes are independently energized with respective voltages sufficient to hold the sample on the chucking surface. The configurations and properties of the dielectric layer can be as summarized above.